This invention relates to liquid jet nozzles, and more particularly to high pressure erosion resistant nozzles for cutting and cleaning, and methods of making the liquid jet nozzles of Nitinol.
Water jet cutting technology was developed in the 1970""s for non-contact cutting of many soft materials that cannot be cut as cleanly or as fast using conventional shearing or sawing techniques. The process uses one or more pumps that pressurize water to a high pressure, typically about 50-60 KPSI, and pass the water through a small orifice, on the order of 2-20 mils, in a nozzle to produce a high velocity water jet.
The water jet cutting technology was improved in the 1980""s with the introduction of abrasive water jet cutting, wherein abrasive particles such as garnet are entrained in the water jet in a mixing chamber and the particle-laden jet is passed through a mixing tube where the abrasive particles are concentrated into the central region of the jet. The addition of abrasive particles greatly improved the to speed of cutting and made it possible for the first time to cut hard materials such as steel plate. Water jet and abrasive water jet are now widely used throughout the world for cutting all manner of materials, and the use is growing.
A recent development is to replace water in the water jet with liquid nitrogen to produce a liquid cutting instrument, sometimes known as a nitrojet. The nitrojet is used for cutting and for cleaning, for example for cleaning old paint that is cracked or chipped from a painted surface such as a ship hull or bulkhead. Cutting nozzles for producing narrow, high velocity streams of liquid are referred to generically herein as xe2x80x9cliquid jet cutting nozzlesxe2x80x9d; cleaning nozzles for producing high velocity streams of liquid, usually in the form of multiple streams, are referred to herein as xe2x80x9cliquid jet cleaning nozzlesxe2x80x9d. Generically, such nozzles are referred to herein as xe2x80x9cliquid jet nozzlesxe2x80x9d.
The most serious problem with existing nozzles for liquid jet instruments is erosion, particularly of the jet orifice and the mixing tube. Liquid moving at high velocity is highly erosive over time and high velocity liquid containing abrasive particles is even more erosive. There are nozzle designs that attempt to concentrate the abrasive particles in the center of the jet, out of contact with the nozzle surfaces, but inevitably some contact occurs which rapidly erodes the mixing tube bore and changes the characteristics of the jet. Typically, the mixing tube must be replaced after as little as one half hour, and usually within 40 hours of operation. The cost of the replacement parts and the necessary down time and labor to install the replacement parts contribute significantly to the operating costs of a liquid jet instrument.
A significant contribution to the erosion of the mixing tube is misalignment of the jet direction with the axis of the mixing tube bore. To achieve alignment, the mixing tube is sometimes pivotally mounted on the nozzle assembly. The principal reason for the misalignment is that the jet emerging from the jewel orifice is not necessarily aimed in the axial direction. The procedure for making the orifice in the jewel and the process for mounting the tiny jewel in the jewel holder are not precise enough to accurately aim the orifice in the desired axial direction. As a consequence, there is little predictability as to the actual exact direction that the jet will have after a jewel has been replaced.
The jewel in the nozzle is typically made of synthetic sapphire about 0.020xe2x80x3-0.050xe2x80x3 thick, and the orifice is drilled with a high speed spinning tungsten rod. However, sapphire is a crystalline material and does not naturally form a smooth cylindrical hole. Accordingly, the walls of the orifice in the sapphire disk, or jewel, are sometimes produced with an irregular profile and rough surfaces. The deviation of the orifice surfaces should be within a certain percentage of the nominal radius, preferably within 1%. A rough and irregular orifice may produce a poorly formed jet which can be ineffective in cutting and may be incapable of being properly aimed to coincide with the axis of the mixing tube, so it may produce rapid wear of the mixing tube. That is one reason for the unpredictable nature of the wear rate of mixing tubes in abrasive water jet cutting apparatus.
The jewel is held in position and centered in the jewel holder by a gasket/seal mounting device. If the orifice in the jewel is not exactly centered or if the mounting device does not center the jewel disc in the jewel holder, the jet will be laterally misaligned. More importantly, if the disc is not seated properly in the holder or if the orifice is not drilled perfectly axial in the disc, the jet will not be axially aligned with the axis of the machine and with the bore of the mixing tube. The result will be rapid wear of the mixing tube.
Pressure fluctuations can occur in a liquid jet apparatus and if back pressure develops, even momentarily, it can blow the jewel out of its seat in the jewel holder. If this occurs, the effective size of the orifice becomes the size of the central hole through the seat, which is far too large to produce an effective cutting jet and requires that the jewel and jewel holder be replaced. Naturally, this entails machine down time while the jewel and jewel holder are replaced. Jewel replacement is costly in terms of replacement cost, labor and machine down-time.
There are several other factors that affect the performance of a liquid jet cutting nozzle. The ratio of jet orifice diameter to length has an effect on repeatably producing nozzle orifices that aim the jet accurately in the axial direction. The orifice surface coefficient of friction with the liquid and the orifice surface finish and orifice surface material toughness and hardness all affect the performance and durability of the liquid jet cutting nozzle.
Accordingly, it is an object of this invention to provide an improved liquid jet nozzle made of Nitinol. Another object of this invention is to provide economical, reliable and repeatable processes for producing liquid jet nozzles made of Nitinol.
These and other objects of the invention are attained in a monolithic Nitinol body having a lead-in channel ending in an integral central web. A small diameter orifice communicates through the web from the lead-in channel to an exit opening in the output side of the body through which the liquid jet exits the body. The orifice is defined by a cylindrical wall having a circular cross section with a smooth, hard, durable surface finish and minimal deviation from nominal circularity.